In the preparation of certain products, such as jet engine discs, by forging of dual or multiple alloy billets, it is preferred that the boundary between the two materials be retained in a relatively straight line parallel with the axis of the disc. In the prior art of forging billets into suitable discs of dual alloy material, the central alloy for the disc is positioned within the center of the billet and the outer disc alloy is a tubular member surrounding the central alloy.
The product of prior art forging of such dual alloy billet is illustrated in the drawings. The interface between the two alloys which has been achieved is shown in FIG. 1 to have a parabolic shape with the central alloy having a very substantially larger diameter at its mid height than at its upper and lower surfaces.
While it has been thought that the secret of being able to have an interface between the two forged alloys which is parallel to the axis is related to the friction between the billet surfaces and the die surfaces, the use of a lubricant such a boron nitride results in a forged disc having an approximately parabolic interface as shown in FIG. 1.
U.S. Pat. No. 3,780,553 discloses the use of lubricant between the forging dies and the billets and of grooves in the surface of the dies which receive lubricant from the surface of the billet as it is forged and make the lubricant available between the die surfaces and the billet throughout the forging process. Such lubrication is not believed to solve the problem addressed by the present application, since the grooves themselves could present additional limiting forces to the movement of the billet during the forging process.